1. Field of the Invention
The invention relates to a technique for providing circuit-switched bearer services in cellular radio systems.
2. Related Art
The capability to support variable-rate circuit-switched bearer services is one of the most important features of the digital mobile communications systems of the third generation. This requirement comes from those applications in which the source data transfer rate varies with time, but not exactly in a "bursty" form. Such a data transfer rate that varies in a "bursty" form with time may normally be supported by packet-switched access bearer services, whereas the variable-rate services still require maintaining the connection in a circuit-switched form. Example applications include variable-rate speech codecs and variable-rate video codecs. Normally, the variable rate of these codecs may be limited to a number of fixed data transfer rate values, but the required speed of variation can be remarkably high, such as from one speech or video frame to the next.
The advantage of these codecs is that the quality of speech and video may be maintained almost as good as if they were operating at the peak data transfer rate, while the average data transfer rate is lower than the peak data transfer rate. This requires, however, the bearer services that support these codecs to be able to vary their transmission rate over the air interface so that the redundant radio channel capacity can be released for other users when the source data transfer rate is reduced from its peak. Otherwise, the use of these variable-rate codecs would not be of any benefit to the system or the network because the factor that limits the capacity of the system is the user peak rate.
The present-day generation of digital mobile communications systems does not support variable-rate circuit-switched bearer services, although in principle any required changes in the user interface rate may be taken into account by means of the handover procedure, during which the connection is handed over to another bearer service, which is often associated with allocating a radio channel resource. This procedure is slow and it cannot be used for supporting any rapid changes in the user source bit rate, such as changes produced e.g. by variable-rate speech or video codec.
A discontinuous transmission (DTX) is another way of achieving the variation of data transfer rate in present-day mobile communications systems. DTX refers to a method by means of which the transmission to the radio path may be interrupted for the duration of pauses occurring in speech. This aims at reducing the consumption of power in transmission, which is absolutely essential to mobile telephones, and the general interference level on the radio path, which has an effect on the capacity of the system. In the pan-European digital mobile communications system, GSM, for instance, a transmitter (a mobile station or base station) usually transmits one traffic burst per each TDMA frame (that is 96 bursts/480 ms) until the speech codec detects a silent period in the speech signal. The transmitter thus transmits only 12 bursts/480 ms. This saves the transmitting power and reduces interference caused to other co-channel users.
Because the radio resources are allocated in mobile communications systems in form of time-slots by the base station controller or the mobile services switching centre, it is not easy to re-use the released radio capacity immediately for other users when the source data transfer rate is reduced. Likewise, it is not easy either to take additional radio capacity into use immediately when the source rate increases. This means that no significant benefit is obtained e.g. from the use of variable-rate speech and video codecs in a mobile communications system from the network point of view.